This invention relates generally to an improved circuit mounting structure for use with an integrated circuit chip, and more particularly, to an improved circuit mounting structure, utilizing resin molding techniques, for use with an integrated circuit block such as those used in electronic watches.
In order to affix an integrated circuit chip to a circuit substrate, when used for example in the circuit block of a watch, various bonding techniques are used, such as: the wire bonding technique, in which an integrated circuit chip is connected directly to a circuit substrate through the use of wires; the gang bonding technique, in which the patterns on a circuit substrate are run over a hole and are utilized as the inner lead wire whereby an integrated circuit is bonded to the circuit substrate; and, the face down bonding technique, in which the circuit contact pad of an integrated circuit chip is directly connected to a circuit pattern provided on a circuit substrate. The invention is directed to improvements in the aforementioned face down bonding technique.
Generally in order to obtain a highly reliable physical and electrical coupling of an integrated circuit element to a circuit substrate, an epoxy resin coating is applied all around an integrated circuit chip through an injection molding process. In practice, the circuit substrate has a hole in that portion of it which is adjacent to the active surface of the integrated circuit chip. A fluid resin material is then introduced into and gated from the substrate hole into the space provided between the integrated circuit chip and the circuit substrate, thereby sealing the entire assembly. When the arrangement of the circuit patterns on the circuit substrate is such that no pattern is run underneath the active surface of the integrated circuit chip, the fluid resin material flows through the hole and completely surrounds the IC unit. A frame is generally mounted on the substrate around the chip to define a boundary for the resin material surrounding the chip. However, this requires an additional part and adversely affects miniaturization. A further problem with the prior art face down bonding technique results from the differences in the thermal coefficient of expansion of the materials. The gap between the chip and substrate must be made undesirably larger to allow for thermal effects, or the chip may be raised and disconnected.
In many instances, it is essential that integrated circuit chips be arranged in a densely packed manner. In these cases it is necessary for circuit patterns, which are formed on the circuit substrate, to be routed beneath the chip. When this occurs, the flow of the fluid resin material as it is gated from the hole is disturbed by the conductive patterns beneath the chip. Consequently, the entire space between the integrated circuit and the circuit substrate is not completely filled with resin material. It has been noted that this is most likely to happen when the space between the active surface of the integrated circuit chip and the conductive pattern on the circuit substrate is small.
Accordingly, it is desirable to provide a circuit mounting structure which eliminates the aforementioned difficulties and allows an integrated circuit chip to be mounted onto a circuit substrate in a close manner while continuing to allow a fluid resin moulding material to flow around, and completely seal the circuit structure.